2024-05-12 02:47:15 -04:00
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# Gregorian Calendar
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2025-06-10 19:08:43 -04:00
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## What is this?
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This is an implementation of the [Gregorian calendar][0], which is the default
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civil calendar in most countries. It is meant to help you cross-reference dates
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by providing the conversions various other calendars and time-keeping systems,
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such as:
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1. the [Julian day number][2] (JDN);
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2. the [Julian calendar][1] date;
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3. the [Mesoamerican Long Count calendar][3] date; and
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4. the [French Republican calendar][4] date.
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You can [see it live here][5]! More conversions will be added in the future.
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## The Gregorian calendar
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The Gregorian calendar itself is based on the Julian calendar, originally
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introduced in 46 BC by Julius Caesar (with aid of Sosigenes of Alexandria).
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Unfortunately, the Julian calendar had leap days every four years without
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question, resulting in the average year having 365.25 days. However, the actual
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tropical year is roughly 365.2422 days, resulting in it gaining a day every 129
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years. This means that seasons drift, starting earlier and earlier in the year.
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In 1582, Pope Gregory XIII decided to fix this drift by reducing the number
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of leap days, motivated by a desire to keep the March equinox on March 21st,
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since that value was hardcoded in the calculation for the date of Easter. To
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achieve this, he made years divisible by 100 but not by 400 non-leap years,
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resulting in 97 leap years every 400 years. To bring the equinox back into
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alignment, October 5th to October 14th in 1582 were deleted, creating the
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Gregorian calendar we use today.
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This website extends the Gregorian calendar indefinitely into the past for
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reference reasons.
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[0]: https://en.wikipedia.org/wiki/Gregorian_calendar
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[1]: https://en.wikipedia.org/wiki/Julian_calendar
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[2]: https://en.wikipedia.org/wiki/Julian_day
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[3]: https://en.wikipedia.org/wiki/Mesoamerican_Long_Count_calendar
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[4]: https://en.wikipedia.org/wiki/French_Republican_calendar
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[5]: https://gcal.qt.ax/
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